Video Restsrta PC

When I click to play video files on the PC or from the web, my Win XP restarts. I DON’T have the option to restart ticked in My Computer->Properties->Advanced->Startup Options either. This is recent but can’t think what has been done recently to change the config’. Have Radeon 9200 128MB video card. All ok in Device Manager, and Event Logs don’t show anything obvious. Ran Virus and Spyware scans, all ok. Any ideas? David